Research in progress will define an animal model for the study of pulmonary infection with L. pneumophila. Initially, the guinea pig will be studied, but hamsters and rats also will be infected. Intratracheal inoculation of L. pneumophila has been shown in preliminary studies to produce pneumonia; but ultimately, a snout-aerosol-exposure system will be used. ID 50 (infectious dose) as measured by histologic and/or immunologic techniques will be determined. Pathologic lesions will be characterized and quantitated. The pathogenesis of the pulmonary lesions will be studied by techniques previously employed by investigators in this group. Alveolar macrophage function will be assessed both in vivo and in vitro. Antibody production in the lung will be evaluated by measuring different classes of immunoglobulins in fluid obtained by bronchoalveolar lavage. The protective effect of either systemic or local humoral immunity will be assessed by challenging survivors of an initial infection a second time. Once the experimental model is characterized fully, modifications can be made that allow selective study of factors such as cigarette smoking, corticosteroid therapy, cytotoxic agents, antibiotics, simultaneous viral infection, etc. on the course of the disease.